High efficiency organic electroluminescent display and method for fabricating the same

ABSTRACT

An organic electroluminescent display comprises: anode electrodes of R, G and B unit pixels formed separate from each other on a substrate; organic thin-film layers of the R, G and B unit pixels formed on the anode electrodes; and a cathode electrode formed over an entire surface of the substrate. The anode electrode of at least one unit pixel, among the R, G and B unit pixels, has a thickness different from anode electrodes of the other unit pixels. The anode electrode of each of the unit pixels comprises a first film having a high reflectivity and a second film for adjusting a work function. The second film of at least one unit pixel, among the unit pixels, has a thickness different from the second films of the other unit pixels. The second film of the R unit pixel is thicker than the second films of the other unit pixels.

CLAIM OF PRIORITY

[0001] This application makes reference to, incorporates the sameherein, and claims all benefits accruing under 35 U.S.C. §119 from myapplication HIGHEFFICIENCY OELD AND METHOD FOR FABRICATING THE SAMEfiled with the Korean Industrial Property Office on 1 May 2003 and thereduly assigned Serial No. 10-2003-0028076.

BACKGROUND OF THE INVENTION

[0002] 1. Technical Field

[0003] The present invention relates to a flat panel display and, moreparticularly, to an organic electroluminescent display and a method forfabricating the same, capable of improving luminous efficiency and colorreproduction by differentiating thicknesses of anode electrodes ofrespective R, G and B unit pixels.

[0004] 2. Prior Art

[0005] Generally, an organic electroluminescent display (OELD) isclassified into a front surface emitting display and a rear surfaceemitting display depending upon the surface of luminescence of light. Onthe basis of a substrate, in case of the rear surface emitting OELD, alight from an electroluminescent layer is emitted and passes through thesubstrate, while in the case of the front surface emitting OELD, lightfrom the electroluminescent layer is emitted without passing through thesubstrate.

[0006] The efficiency of the rear surface emitting structure isdetermined depending upon optical characteristics of a reflection filmand a transmissive anode electrode, and electrical characteristics of anorganic thin-film layer including an electroluminescent layer. A holetransporting layer is formed so as to be thicker than an electrontransporting layer since maximum constructive interference in opticalcharacteristics is generated at a thickness of ¼ wavelength of lightemitted. The mobility of the hole transporting layer is faster than thatof the electron transporting layer in terms of the electricalcharacteristics. Therefore, the thickness of the electroluminescentlayer presenting the maximum efficiency is determined when a full colorOELD of the rear surface emitting structure is manufactured.

[0007] On the other hand, the thicknesses of the hole transportinglayer, the electroluminescent layer and the electron transporting layerlocated between a reflective anode electrode and a transflective cathodeelectrode for measuring the optical thickness, and the electricalthickness in the front surface emitting OELD, are determined differentlyfrom the rear surface emitting OELD.

[0008] There have been prior attempts to obtain the maximum efficiencyand the highest color purity by controlling the thicknesses of a holeinjecting and transporting layer, an electroluminescent layer and anelectron transporting layer making up the organic thin-film layerinterposed between the anode electrode and the cathode electrode.Japanese Patent Registration No. 2846571 has disclosed technology in thearea of the rear surface emitting organic electroluminescent displaycapable of obtaining a high color purity and efficiency by setting anoptical film thickness of the anode electrode, the cathode electrode andthe organic thin-film layers between the anode and cathode electrodes toachieve a peak in the strength of light emitted from theelectroluminescent layer. Further, Japanese Laid-open Patent PublicationNo. 2000-323277 has disclosed technology in the area of the rear surfaceemitting organic electroluminescent display capable of obtaining a highefficiency and color purity by differently forming the thickness ofthin-film layers, except for the electroluminescent layer, among theorganic thin-film layers interposed between the anode electrode and thecathode electrode, depending upon the R, G and B unit pixels.

[0009] However, the front surface emitting organic electroluminescentdisplay has a problem in that, although the thickness of the thin-filmlayers is set to ¼ wavelength of a desired light, it is difficult toobtain a desired efficiency and color purity since theelectroluminescent layer is located between reflection sections of thereflective anode electrode and the semitransmissive cathode electrode.

[0010] On the other hand, in the front surface emittingelectroluminescent display, U.S. Patent Application assigned Ser. No.10/385,453 entitled “Organic Electroluminescent Device EmployingMulti-Layered Anode”, by Kwanhee Le, filed in the United States Patent &Trademark Office on the 12^(th) day of March 2003, has disclosedtechnology capable of improving luminescence characteristics by formingan anode electrode of a multi-layered structure.

[0011] Anode electrodes of the respective R, G and B unit pixels areformed on an insulating substrate. The anode electrodes include a firstanode and a second anode. A pixel defining layer is formed to exposeportions of the anode electrodes, thereby forming apertures of therespective R, G and B unit pixels. Organic thin film layers of the R, G,and B unit pixels, including R, G, B electroluminescent layers, areformed on the anode electrodes of the R, G and B unit pixels,respectively, in the apertures. A semitransmissive cathode electrode isformed on the entire surface of the substrate.

[0012] The front surface emitting organic electroluminescent display hasformed thereon anode electrodes with a 2-layered structure, employing afirst anode electrode as a metal film having a high reflectivity, and asecond anode electrode as a metal film capable of conforming with a workfunction, thereby improving luminous efficiency by increasingreflectivity and a hole injecting characteristic.

[0013] However, in the front surface emitting organic electroluminescentdisplay, all of the second anode electrodes of the respective R, G and Bunit pixels have the same thickness. Therefore, it is impossible toobtain desired color reproduction and efficiency, since the first anodeelectrode with a good reflectivity and the semitransmissive cathodeelectrode have respective lengths of optical constructive interferencewhich are different from each other.

SUMMARY OF THE INVENTION

[0014] Therefore, to solve the problem described hereinabove, an objectof the present invention is to provide an organic electroluminescentdisplay, and a method for fabricating the same, capable of obtainingmaximum color reproduction and highest efficiency.

[0015] Another object of the present invention is to provide an organicelectroluminescent display, and a method for fabricating the same,capable of obtaining a desired color reproduction and illuminousefficiency by differently forming the thicknesses of anode electrodes ofrespective R, G and B unit pixels.

[0016] Still another object of the present invention is to provide anorganic electroluminescent display, and a method for fabricating thesame, capable of improving color reproduction and luminous efficiency byusing a simple process of differently forming the thicknesses of anodeelectrodes of respective R, G and B unit pixels without an additionalmask process.

[0017] To accomplish the above-mentioned objects, the present inventionprovides an organic electroluminescent display, comprising: anodeelectrodes of R, G and B unit pixels formed so as to be separated fromeach other on a substrate; organic thin-film layers of the R, G and Bunit pixels formed on the anode electrodes; and a cathode electrodeformed on an entire surface of the substrate; wherein an anode electrodeof at least one unit pixel of the R, G and B unit pixels has a thicknessdifferent from anode electrodes of the other unit pixels.

[0018] In an embodiment of the invention, each of the anode electrodesof the unit pixels comprises a first film having a high reflectivity anda second film for adjusting a work function, and the second film of atleast one unit pixel has a thickness different from the second films ofthe other unit pixels. The second film of the R unit pixel is thickerthan the second films of the other unit pixels.

[0019] In a preferred embodiment of the invention, the thickness of thesecond film of the R unit pixel is in a range from 250 to 450 Å or from700 to 750 Å, and the thicknesses of the second films of the G and Bunit pixels are in a range from 50 to 150 Å. The thickness of the secondfilm of the R unit pixel is in a range from 250 to 450 Å or from 700 to750 Å, the thickness of the second film of the G unit pixel is in arange from 200 to 300 Å, and the thickness of the second film of the Bunit pixel is in a range from 50 to 150 Å.

[0020] In a further embodiment of the invention, in order to obtainmaximum efficiency, in the R, G and B unit pixels, the thickness of thesecond film of the R unit pixel is 375 Å, the thickness of the secondfilm of the G unit pixel is 250 Å, and the thickness of the second filmof the B unit pixel is 125 Å. Moreover, in order to obtain maximum colorreproduction, the thickness of the second film of the R unit pixel is750 Å, the thickness of the second film of the G unit pixel is 250 Å,and the thickness of the second film of the B unit pixel is 125 Å.

[0021] The first film of each of the unit pixels is composed of Al, Agor an alloy film thereof, and the second films are composed of ITO orIZO.

[0022] Further, the present invention provides an organicelectroluminescent display comprising a number of pixels, each includingat least an anode electrode, wherein the anode electrodes of adjacentpixels among the number of pixels have different thicknesses withrespect to each other.

[0023] In an embodiment of the invention, the anode electrode of each ofthe pixels comprises a first film having a high reflectivity and asecond film for adjusting a work function, the second films of the anodeelectrodes of adjacent pixels having a thickness different from eachother.

[0024] In addition, the present invention provides a method forfabricating an organic electroluminescent display, comprising the stepsof: forming first anodes of R, G and B unit pixels on a substrate;forming an anode electrode of the R unit pixel by forming a second anodeof the R unit pixel on the first anode of the R unit pixel; forminganode electrodes of the G and B unit pixels by forming second anodes ofthe G and B unit pixels on the first anodes of the G and B unit pixels;forming organic thin-film layers on the anode electrodes of the R, G andB unit pixels, respectively; and forming a cathode electrode on anentire surface of the substrate; wherein a second anode of at least oneunit pixel, among the R, G and B unit pixels, has a thickness differentfrom the thicknesses of the second anodes of the other unit pixels.

[0025] Further, the present invention includes a method for fabricatingan organic electroluminescent display, comprising the steps of: formingsequentially a first anode electrode material and a second anodeelectrode material of the R, G and B unit pixels on a substrate; forminganode electrodes of the R, G and B unit pixels, each including a firstanode and a second anode, by etching the first and the second anodeelectrode materials; forming organic thin-film layers on the anodeelectrodes of the R, G and B unit pixels, respectively; and forming acathode electrode on an entire surface of the substrate; wherein asecond anode of at least one unit pixel, among the R, G and B unitpixels, has a thickness different from the thicknesses of second anodesof the other unit pixels.

[0026] Further, the present invention includes a method for fabricatingan organic electroluminescent display, comprising the steps of: formingfirst anodes of R, G and B unit pixels on a substrate; forming a secondanode electrode material on an entire surface of the substrate; etchingthe second anode electrode material to form second anodes on the firstanodes of the R, G and B unit pixels, respectively, and to form anodeelectrodes of the R, G and B unit pixels; forming organic thin-filmlayers on the anode electrodes of the R, G and B unit pixels,respectively; and forming a cathode electrode on an entire surface ofthe substrate; wherein a second anode of at least one unit pixel, amongthe R, G and B unit pixels, has a thickness different from thethicknesses of second anodes of the other unit pixels.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] A more complete appreciation of the invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings in which like reference symbols indicate the same or similarcomponents, wherein:

[0028]FIG. 1 is a cross-sectional view of a prior art organicelectroluminescent display;

[0029]FIG. 2 is a cross-sectional view of an organic electroluminescentdisplay in accordance with an embodiment of the present invention;

[0030]FIGS. 3A thru 3E are cross-sectional views of processes forfabricating an organic electroluminescent display in accordance with afirst embodiment of the present invention;

[0031]FIGS. 4A thru 4D are cross-sectional views of processes forfabricating an organic electroluminescent display in accordance with asecond embodiment of the present invention;

[0032]FIGS. 5A thru 5D are cross-sectional views of processes forfabricating an organic electroluminescent display in accordance with athird embodiment of the present invention; and

[0033]FIGS. 6A thru 6C are spectrums of R, G and B colors, respectively,in an organic electroluminescent display in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0034] Hereinafter, a detailed description of preferred embodiments ofthe present invention will be apparent in connection with theaccompanying drawings.

[0035]FIG. 1 is a cross-sectional view of a prior art organicelectroluminescent display. As shown in FIG. 1, anode electrodes 110,120 and 130 of the respective R, G and B unit pixels are formed on aninsulating substrate 100. The anode electrodes 110, 120 and 130 includefirst anodes 111, 123, 125 and second anodes 121, 123 and 125,respectively. A pixel defining layer 140 is formed to expose portions ofthe anode electrodes 110, 120 and 130, thereby forming apertures 141,143 and 145 of the respective R, G and B unit pixels. Organic thin filmlayers 150, 160 and 170 of the R, G, and B unit pixels, including R, G,B electroluminescent layers, are formed on the anode electrodes 110, 120and 130 of the R, G and B unit pixels in the apertures 141, 143 and 145,respectively. A semitransmissive cathode electrode 180 is formed on anentire surface of the substrate.

[0036] The front surface emitting organic electroluminescent display hasanode electrodes with a 2-layered structure, employing a first anodeelectrode as a metal film having a high reflectivity, and a second anodeelectrode as a metal film capable of conforming with a work function,thereby improving luminous efficiency by increasing reflectivity and ahole injecting characteristic.

[0037] However, in the front surface emitting organic electroluminescentdisplay, all of the second anode electrodes of respective R, G and Bunit pixels have the same thickness. Therefore, it is impossible toobtain desired color reproduction and efficiency, since the first anodeelectrode with a good reflectivity and the semitransmissive cathodeelectrode have a different lengths of optical constructive interferencewith respect to each other

[0038]FIG. 2 is a cross-sectional view of an organic electroluminescentdisplay in accordance with an embodiment of the present invention.

[0039] Referring to FIG. 2, a buffer layer 210 is formed on atransparent insulation substrate 200, and semiconductor layers 220, 230and 240 of R, G and B unit pixels provided with source/drain regions 221and 225, 231 and 235, and 241 and 245 are formed on the buffer layer210, respectively. Gates 261, 263 and 265 of the respective unit pixelsare formed on a gate insulating film 250, and source/drain electrodes281 and 285, 291 and 295, and 301 and 305 of the respective unit pixelsconnected to the source/drain regions 221 and 225, 231 and 235, and 241and 245 of the respective unit pixels through contact holes (not shown)are formed on an inter-layer insulating film 270.

[0040] Further, anode electrodes 320, 330 and 340 of the R, G and B unitpixels are formed on a planarization film 310 so as to be connected toone of the source/drain electrodes of thin-film transistors of the unitpixels, for example, drain electrodes 285, 295 and 305 through via-holes311, 313 and 315, respectively. At this time, each of the anodeelectrodes 320, 330 and 340 of the R, G and B unit pixels includes afirst anode 321, 331 and 341 having a high reflectivity and a secondanode 325, 335 and 345 for adjusting a work function, and the secondanode of at least one unit pixel among the R, G and B unit pixels, isformed to have a thickness different from the thicknesses of the secondanodes of the other unit pixels.

[0041] In an embodiment of the present invention, the second anode 325of the R unit pixel is formed so as to be thicker than the second anodes335 and 345 of the G and B unit pixels, and the thicknesses of thesecond anodes 335 and 345 of the G and B unit pixels are formed with thesame thickness. Otherwise, the second anode 325 of the R unit pixel isformed so as to be thicker than the second anodes 335 and 345 of the Gand B unit pixels, and the second anode 335 of the G unit pixel isformed so as to be thicker than the second anode 345 of the B unitpixel.

[0042] On the planarization film 310, a pixel defining layer 360 forseparating the anode electrodes 320, 330 and 340 of the respective unitpixels is formed. The pixel defining layer 360 is provided withapertures 371, 373 and 375 for exposing portions of the anode electrodes320, 330 and 340, respectively. The pixel defining layer 360 may employa conventional thermosetting resin or a photosensitive resin. Organicthin-film layers 381, 383 and 385 of the respective unit pixels areformed on the anode electrodes 320, 330 and 340 of the respective unitpixels in the apertures 371, 373 and 375, and a cathode electrode 390 isformed on an entire surface of the substrate. The organic thin-filmlayers 381, 383 and 385 of the respective unit pixels comprise anelectroluminescent layer of the respective unit pixels, including atleast one of a hole injecting layer, a hole transporting layer, a holeblocking layer, an electron injecting layer and an electron transportinglayer.

[0043] The organic electroluminescent display in accordance with thepresent invention is capable of obtaining the highest luminousefficiency by forming the thicknesses of the second anodes 325, 335 and345 of the anode electrodes 320, 330 and 340, respectively, of the R, Gand B unit pixels, respectively, so as to be different from each otherdepending upon the unit pixels.

[0044] Hereinafter, a detailed description of a method for fabricatingan organic electroluminescent display provided with anode electrodeshaving a thickness different from each other, depending upon the R, Gand B unit pixels, in accordance with the present invention, will beprovided. In the method for fabricating the organic electroluminescentdisplay in accordance with the present invention, since the processprior to formation of the anode electrode is similar to a conventionalmethod, hereinafter, only the process of forming the anode electrodeshaving thicknesses different from each other, depending upon the R, Gand B unit pixels, and the following processes, will be described.

[0045] A description of the method for fabricating the organicelectroluminescent display in accordance with a first embodiment of thepresent invention will be given with reference to FIGS. 3A to 3E.

[0046] Referring to FIG. 3A, a first anode electrode material 410 isformed to have a thickness of 2000 Å; with a metal film such as Al, Agor an alloy thereof having a high reflectivity, on a transparentinsulating substrate 400, such as a glass substrate, by using a DCsputter. Referring to FIG. 3B, the first anode electrode material 410 ispatterned to form first anodes 421, 423 and 425 with all of the R, G andB unit pixels having the same thickness.

[0047] Referring to FIG. 3C, a second anode electrode material isdeposited with a predetermined thickness on the entire surface of thesubstrate with a material having a suitable work function, for example,ITO or IZO, and is patterned to form a second anode 431 of the R unitpixel on the first anode 421 of the R unit pixel only. The second anode431 of the R unit pixel is formed with a thickness of 250˜450 Å or700˜750 Å, preferably 375 Å.

[0048] Referring to FIG. 3D, material the same as the second anodeelectrode material of the R unit pixel (for example, ITO or IZO) isdeposited with a thickness of 50˜150 Å on the entire surface of thesubstrate, and is patterned to form second anodes 433 and 435 of the Gand B unit pixels on the first anodes 423 and 425, respectively.Therefore, the anode electrodes 441, 443 and 445 of the R, G and B unitpixels, including the first anodes 421, 423 and 435 having a highreflectivity and the second anodes having suitable work functions 431,433 and 435, are formed.

[0049] On the other hand, by adding a mask process instead of equallyforming the thicknesses of the second anodes of the G and B unit pixels,the second anode 433 of the G unit pixel is formed with a thickness of200˜300 Å, preferably 250 Å, and the second anode 435 of the B unitpixel is formed with a thickness of 50˜150 Å, preferably 125 Å, therebyforming the second anodes 431, 433 and 435 of the R, G and B unitpixels, respectively, with different thicknesses from each other.

[0050] Referring to FIG. 3E, after the organic insulating film composedof a thermosetting resin or a photosensitive resin is deposited on theentire surface of the substrate, it is patterned by a conventionalmethod to form a pixel defining layer 450 for separating the anodeelectrodes 441, 443 and 445 of the respective unit pixels. After thecompletion of forming the pixel defining layer 450, the layer 450 issequentially cleansed by using water, isopropyl alcohol and acetone.Then, it is cleansed by using a UV/O3 cleanser. At this time,luminescence sections of the anode electrodes of the respective unitpixels are opened, depending upon the forming of the pixel defininglayer 450, and have a pattern dimension of 2 mm×2 mm.

[0051] Subsequently, while not shown, in the process of forming theorganic thin-film layer on the anode electrodes 441, 443 and 445,corresponding organic films from among the hole injecting layer, thehole transporting layer, the electroluminescent layer, the hole blockinglayer, and the electron transporting layer of the R, G and B unit pixelsare sequentially formed. The hole injecting layer is formed with athickness of 250 Å by using an IDE 406 of Idemitsu Co. with vacuumdeposition, and the hole transporting layer is formed with a thicknessof 100 Å by vacuum deposition with a speed of 0.1 nm/sec by usingNBP{N,N′-di(naphthalene-1-yl) -N,N′-diphenyl-benzidine}.

[0052] Continuously, CBP {4,4′-bis(carozol-9-yl)-biphenyl} andphosphorescene red are heat deposited with a 100: 12 mixing weight ratioto form the electroluminescent layer of the R unit pixel with athickness of 300 Å. The CBP and IrPPy{tris(phenylpyridine)Iridium} areheat deposited with a 100:5 mixing weight ratio to form theelectroluminescent layer of the G unit pixel with a thickness of 250 Å.A blue host and a blue dopant are heat deposited with a 100:4 mixingweight ratio to form the electroluminescent layer of the B unit pixelwith a thickness of 150 Å.

[0053] Next, BAlq is deposited with a thickness of 50 Åto form a holebarrier layer, and Alq3 {tri(8-quinolinolate)-aluminium} is vacuumdeposited to form the electron transporting layer with a thickness 250Å. Magnesium (Mg) and silver (Ag) are heat deposited with 10:1 to 30:1mixing weight ratio to form a semitransmissive cathode having athickness of 50˜150 Å, preferably 100 Å. Then, IZO is deposited usingsputter, under a vacuum condition, of a speed of 0.2 nm/sec and apressure of 1×10{circumflex over ( )}-5 Pa to form a transmissivecathode electrode.

[0054] Finally, a passivation layer is formed to prevent oxygen andmoisture from penetrating to the exterior, and to protect the innerorganic thin-film layer. An encapsulation substrate is attached andencapsulated by using a UV adhesive under a nitrogen gas atmosphere andan anhydrous condition. Then, the front surface emitting organicelectroluminescent display is manufactured by thermosetting fr 1 hour ata temperature of about 70° C.

[0055] A description of the method for fabricating the organicelectroluminescent display in accordance with a second embodiment of thepresent invention will be provided with reference to FIGS. 4A to 4D. Thefabricating method in accordance with the second embodiment of thepresent invention is accomplished with the same condition as the firstembodiment, except that the mask process is reduced than the firstembodiment by using a half-tone mask.

[0056] Referring to FIG. 4A, a first anode material 510 and a secondanode material 520 are sequentially deposited on an insulating substrate500. Referring to FIG. 4B, a photosensitive film 530 is coated on thesecond anode electrode material. Then, a photo process is performed byusing the half-tone mask 540. The half-tone mask 540 includes: a lightblocking section 541 for blocking light entirely, corresponding to aportion forming the anode electrode of the R unit pixel;halftransmitting sections 543 and 545 for transmitting a portion oflight, corresponding to portions forming the anode electrodes of the Gand B unit pixels; and a transmitting section 547 for transmitting lightentirely.

[0057] Referring to FIG. 4C, photosensitive film patterns 531, 533 and535 having thicknesses different from each other, depending upon the R,G and B unit pixels, are formed by a photo process using the half-tonemask 540. The photosensitive film pattern 531 of the R unit pixel isformed so as to be thicker than the photosensitive film patterns 533 and535 of the G and B unit pixels, and the photosensitive film patterns 533and 535 of the G and B unit pixels have the same thickness.

[0058] Referring to FIGS. 4C and 4D, the first and the second anodeelectrode materials 510 and 520 are patterned by using thephotosensitive film patterns 531, 533 and 535 to form the anodeelectrodes 551, 553 and 555 having thicknesses different from eachother, depending upon the R, G and B unit pixels. At this time, thefirst anodes 511, 513 and 515 from among the anode electrodes 551, 553and 555 of the R, G and B unit pixels have the same thickness, and thesecond anodes 521, 523 and 525 have thicknesses different from eachother, depending upon the thickness difference of the photosensitivefilm patterns 531, 533 and 535.

[0059] That is to say, the second anode 521 of the R unit pixel isformed so as to be thicker than the second anodes 523 and 525 of the Gand B unit pixels, and the second anodes 523 and 525 of the G and B unitpixels are formed so as to have the same thickness. In the secondembodiment, the second anodes 523 and 525 of the G and B unit pixels maybe formed so as to have different thicknesses by making the thicknessesof the half transmitting patterns 543 and 545 of the half-tone mask 540different when the photosensitive film 530 is patterned using thehalf-tone mask shown in FIG. 4B.

[0060] A description of the method for fabricating the organicelectroluminescent display in accordance with a third embodiment of thepresent invention will be provided with reference to FIGS. 5A to 5D. Thefabricating method in accordance with the third embodiment of thepresent invention is accomplished with the same condition as the firstembodiment, except that the mask process is reduced relative to thefirst embodiment by using a half-tone mask.

[0061] Referring to FIG. 5A, a first anode electrode material isdeposited and patterned on an insulating substrate 600 to form the firstanodes 611, 613 and 615 of the R, G and B unit pixels having the samethickness. Subsequently, the second anode electrode material 620 isdeposited on the substrate 600 including the first anodes 611, 613 and615.

[0062] Referring to FIG. 5B, a photosensitive film 630 is coated on thesecond anode electrode material. Then, a photo process is performed byusing the half-tone mask 640. The half-tone mask 640 includes: a lightblocking section 641 for blocking light entirely, corresponding to afirst anode 611 of the R unit pixel; half transmitting sections 643 and645 for transmitting a portion of light, corresponding to second anodes613 and 615 of the G and B unit pixels; and a transmitting section 647for transmitting light entirely.

[0063] Referring to FIG. 5C, photosensitive film patterns 631, 633 and635 having thicknesses different from each other, depending upon the R,G and B unit pixels, are formed by a photo process using the half-tonemask 640. The photosensitive film pattern 631 of the R unit pixel isformed so as to be thicker than the photosensitive film patterns 633 and635 of the G and B unit pixels, and the photosensitive film patterns 633and 635 of the G and B unit pixels have the same thickness.

[0064] Referring to FIGS. 5C and 5D, the second anode electrode material620 is patterned by using the photosensitive film patterns 631, 633 and635 to form the second anodes 621, 623 and 625, respectively, havingthicknesses different from each other depending upon the R, G and B unitpixels. That is to say, the second anode 621 of the R unit pixel isformed so as to be thicker than the second anodes 623 and 625 of the Gand B unit pixels, and the second anodes 623 and 625 of the G and B unitpixels are formed so as to have the same thickness.

[0065] Therefore, the anode electrodes 651, 653 and 655 of the R, G andB unit pixels, respectively, are composed of the first anodes 611, 613and 615, respectively, having the same thickness, and the second anodes521, 523 and 525, respectively, having thicknesses different from eachother, thereby having thicknesses different from each other dependingupon the R, G and B unit pixels. In the third embodiment, the secondanodes 623 and 625 of the G and B unit pixels, respectively, may beformed to have different thicknesses by making the thicknesses of thehalf transmitting patterns 643 and 645 different from each other whenthe photosensitive film 630 is patterned using the half-tone mask shownin FIG. 5B.

[0066] Table 1, Table 2 and Table 3 represent efficiency, brightness andchromaticity coordinates of the R, G and B unit pixels depending uponthe thicknesses of the second anodes in accordance with the presentinvention. TABLE 1 R unit pixel thickness efficiency brightnesschromaticity coordinates (Å) (Cd/A) (Lm/W) (CIE_x, CIE_y) 125 5.92 3.600.62, 0.38 375 12.03 7.76 0.64, 0.35 500 0.44 0.19 0.68, 0.31 750 5.593.43 0.67, 0.33

[0067] TABLE 2 G unit pixel thickness efficiency brightness chromaticitycoordinates (Å) (Cd/A) (Lm/W) (CIE_x, CIE_y) 125 32.33 17.26 0.23, 0.68375 10.85 4.85 0.45, 0.53 500 0.23 0.06 0.32, 0.40 750 3.20 1.37 0.52,0.47

[0068] TABLE 3 B unit pixel thickness efficiency brightness chromaticitycoordinates (Å) (Cd/A) (Lm/W) (CIE_x, CIE_y) 125 4.24 2.81 0.13, 0.14375 3.28 1.95 0.21, 0.49 500 0.17 0.07 0.18, 0.08 750 1.46 0.73 0.33,0.53

[0069] From Table 1, the R unit pixel has the highest efficiency andbrightness when the thickness is 375 Å, and the chromaticity coordinateshas the highest value when the thickness is 750 Å. Therefore,considering all of the efficiency, brightness and chromaticitycoordinates, it is preferable that the second anode of the anodeelectrode in the R unit pixel be formed with a thickness of 375 Å.

[0070] From Table 2, since the G unit pixel has the highest efficiencyand brightness when the thickness is 125 Å, and the chromaticitycoordinates are also stable, it is preferable that the second anode ofthe anode electrode in the G unit pixel be formed with a thickness of125 Å.

[0071] From Table 3, since the B unit pixel has the highest efficiencyand brightness when the thickness is 125 Å, and the chromaticitycoordinates are also stable, it is preferable that the second anode ofthe anode electrode in the B unit pixel be formed with a thickness of125 Å.

[0072]FIGS. 6A to 6C are spectrums of R, G and B, respectively, in anorganic electroluminescent display in accordance with the presentinvention.

[0073] In accordance with the embodiment of the present invention, asthe thicknesses of the electrodes for conforming the work function amongthe anode electrodes of the multi-layered structure are formeddifferently from each other depending upon the R, G and B unit pixels,the respective unit pixels are capable of obtaining the highestefficiency. Further, the R and B unit pixels are capable of obtainingboth the highest efficiency and the maximum color purity when a fullcolor device is embodied.

[0074] In addition, when the anode electrodes having differentthicknesses are formed, since an additional process is excluded byadopting the half-tone mask, effects of a process simplification andyield improvement are produced.

[0075] While this invention has been described in connection with whatis presently considered to be the most practical and preferredembodiment, it is to be understood that the invention is not limited tothe disclosed embodiment, but on the contrary, it is intended to covervarious modification within the spirit and the scope of the appendedclaims.

What is claimed is:
 1. An organic electroluminescent display,comprising: anode electrodes of R, G and B unit pixels disposed on asubstrate and separated from each other; organic thin-film layers of theR, G and B unit pixels disposed on the anode electrodes; and a cathodeelectrode disposed over an entire surface of the substrate, wherein ananode electrode of at least one unit pixel of the R, G and B unit pixelshas a thickness different from thicknesses of anode electrodes of otherunit pixels of the R, G and B unit pixels.
 2. The organicelectroluminescent display according to claim 1, wherein the anodeelectrode of the R unit pixel is thicker than the anode electrodes ofthe other unit pixels.
 3. The organic electroluminescent displayaccording to claim 1, wherein the anode electrode of each of the unitpixels includes a first film having a high reflectivity and a secondfilm for adjusting a work function, and wherein the second film of saidat least one unit pixel of the R, G and B unit pixels has a thicknessdifferent from thicknesses of the second films of the other unit pixelsof the R, G and B unit pixel.
 4. The organic electroluminescent displayaccording to claim 3, wherein the second film of the R unit pixel isthicker than the second films of the other unit pixels.
 5. The organicelectroluminescent display according to claim 3, wherein a thickness ofthe second film of the R unit pixel is in a range of one of 250 to 450 Åand 700 to 750 Å, and thicknesses of the second films of the G and Bunit pixels are in a range of 50 to 150 Å.
 6. The organicelectroluminescent display according to claim 3, wherein a thickness ofthe second film of the R unit pixel is in a range of one of 250 to 450 Åand 700 to 750 Å, a thickness of the second film of the G unit pixel isin a range of 200 to 300 Å, and a thickness of the second film of the Bunit pixel is in a range of 50 to 150 Å.
 7. The organicelectroluminescent display according to claim 3, wherein a thickness ofthe second film of the R unit pixel is substantially 375 Å, a thicknessof the second film of the G unit pixel is substantially 250 Å, and athickness of the second film of the B unit pixel is substantially 125 Å,whereby maximum efficiency is obtained in the R, G and B unit pixels. 8.The organic electroluminescent display according to claim 3, wherein athickness of the second film of the R unit pixel is substantially 750 Å,a thickness of the second film of the G unit pixel is substantially 250Å, and a thickness of the second film of the B unit pixel issubstantially 125 Å, whereby maximum color reproduction is obtained inthe R, G and B unit pixels.
 9. The organic electroluminescent displayaccording to claim 3, wherein the first film of each of the unit pixelscomprises one of Al, Ag and an alloy film thereof, and the second filmcomprises one of ITO and IZO.
 10. An organic electroluminescent displaycomprising: a plurality of pixels, each including at least an anodeelectrode; wherein anode electrodes of adjacent pixels have differentthicknesses relative to each other.
 11. The organic electroluminescentdisplay according to claim 10, wherein the anode electrode of each ofthe pixels includes a first film having a high reflectivity and a secondfilm for adjusting a work function, and wherein the second films of theanode electrodes of adjacent pixels have different thicknesses relativeto each other.
 12. A method for fabricating an organicelectroluminescent display, comprising the steps of: disposing firstanodes of R, G and B unit pixels on a substrate; forming an anodeelectrode of the R unit pixel by disposing a second anode of the R unitpixel on the first anode of the R unit pixel; forming anode electrodesof the G and B unit pixels by disposing second anodes of the G and Bunit pixels on the first anodes of the G and B unit pixels,respectively; disposing respective organic thin-film layers on the anodeelectrodes of the R, G and B unit pixels; and disposing a cathodeelectrode over an entire surface of the substrate, wherein the secondanode of at least one unit pixel of the R, G and B unit pixels has athickness different from thicknesses of the second anodes of other unitpixels of the R, G and B unit pixels.
 13. The method according to claim12, wherein the second film of the R unit pixel is thicker than thesecond films of the other unit pixels of the R, G and B unit pixels. 14.The method according to claim 12, wherein a thickness of the second filmof the R unit pixel is in a range of one of 250 to 450 Å and 700 to 750Å, a thickness of the second film of the G unit pixel is in a range ofone of 50 to 150 Å and 200 to 300 Å, and a thickness of the second filmof the B unit pixel is in a range of 50 to 150 Å.
 15. A method forfabricating an organic electroluminescent display, comprising the stepsof: disposing sequentially a first anode electrode material and a secondanode electrode material of R, G and B unit pixels on a substrate;etching the first and second anode electrode materials to form anodeelectrodes of the R, G and B unit pixels, each including a first anodeand a second anode; disposing respective organic thin-film layers on theanode electrodes of the R, G and B unit pixels; and disposing a cathodeelectrode over an entire surface of the substrate, wherein a secondanode of at least one unit pixel of the R, G and B unit pixels has athickness different from thicknesses of second anodes of the other unitpixels of the R, G and B unit pixels.
 16. The method according to claim15, wherein the second film of the R unit pixel is thicker than thesecond films of the other unit pixels.
 17. The method according to claim15, wherein a thickness of the second film of the R unit pixel is in arange of one of 250 to 450 Å and 700 to 750 Å, a thickness of the secondfilm of the G unit-pixel is in a range of one of 50 to 150 Å and 200 to300 Å, and a thickness of the second film of the B unit pixel is in arange of 50 to 150 Å.
 18. A method for fabricating an organicelectroluminescent display, comprising the steps of: disposing firstanodes of R, G and B unit pixels on a substrate; disposing a secondanode electrode material over an entire surface of the substrate;etching the second anode electrode material to form respective secondanodes on the first anodes of the R, G and B unit pixels, therebyforming respective anode electrodes of the R, G and B unit pixels;disposing organic thin-film layers on the respective anode electrodes ofthe R, G and B unit pixels; and disposing a cathode electrode over anentire surface of the substrate; wherein a second anode of at least oneunit pixel of the R, G and B unit pixels has a thickness different fromthicknesses of second anodes of the other unit pixels of the R, G and Bunit pixels.
 19. The method according to claim 18, wherein the secondfilm of the R unit pixel is thicker than the second films of the otherunit pixels.
 20. The method according to claim 18, wherein a thicknessof the second film of the R unit pixel is in a range of one of 250 to450 Å and 700 to 750 Å, a thickness of the second film of the G unitpixel is in a range of one of 50 to 150 Å and 200 to 300 Å, and athickness of the second film of the B unit pixel is in a range of 50 to150 Å.